1. shallow foundationsocw.snu.ac.kr/sites/default/files/note/459.pdf · 2018. 1. 30. · nr by...

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1. Shallow Foundations

1) General

* Shallow Foundations: Foundations that transmit structural loads to the near-

surface soils. (Spread footing foundation + Mat foundation)

Df/B≤1 (by Terzaghi) → Later Df/B≤3∼4

* Requirements to satisfactory foundations

i) Safe against shear failure (bearing capacity failure).

ii) Should not undergo excessive displacements.

(settlements ←differential settlements)

iii) Consideration on the any future influences which could adversely affect

its performance. (frost action, scouring of pier foundations of bridge,

… )

* Types of shallow foundation

� Spread footings

- Spread footing foundations: An enlargement at the bottom of a column

or bearing wall that spreads the applied structural loads over a

sufficiently large soil area.

i) Square spread footings : Supporting a single centrally-supported

column.

ii) Rectangular spread footings : In cases that obstructions prevent

Df

B

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construction of a square footing with a sufficiently large base area

and large moment loads are present.

iii) Circular spread footings : Supporting a single centrally-supported

column, but less common than square footing. (flagpoles).

iv) Continuous spread footings(Strip footings) : Used to support

bearing walls.

v) Combined footing : When columns are located too close together

for each to give its own footing.

vi) Strap footing with a grade beam : Provides the necessary moment

resistance in the exterior footing with eccentric load and a more

rigid foundation system.

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� Raft (mat) foundation

- Mat foundations (Raft foundations): A very large spread footing that

usually encompasses the entire footprint of the structure.

- The advantages of the mat foundation over individual spread footings

i) Spreads the structure load over a larger area, thus reduces bearing

pressure.

ii) Provides much more structural rigidity and thus reduces the

potential for excessive differential settlements.

iii) Is easier to water proof.

iv) Has a greater weight and thus is able to resist greater uplift

pressure.

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2) Bearing Pressure

* Bearing Pressure : the contact forces per unit area along the bottom of the

footing.

* The actual distribution of the bearing pressure depends on ;

- Structural rigidity of the footing.

- Stress-strain properties of the soil.

- Eccentricity of the applied load.

- Magnitude of the applied moment.

- Roughness of the bottom of the footing.

* Perfectly flexible footings

Bend but maintain a uniform bearing pressure.

* Perfectly rigid footings

Settle uniformly but have variations in the bearing pressure.

Close to behaviors of the real footing. (especially for spread footing

but not for raft foundation)

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For bearing capacity and settlement analysis, we assume the uniform

pressure with rigid footings. (Error is not significant for spread footing.)

* The footing subjected to eccentric and/or moment loads.

The bearing pressure is biased toward one side.

< Distribution of bearing pressure along the base of spread footing subjected to eccentric

and/or moment loads : (a) actual distribution (b) simplified distribution >

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* Presumptive Bearing Pressures

- Presumptive bearing pressures are allowable bearing pressures based on

experience and expressed as a function of soil type. (An attempt to

control excessive settlements.)

Give quick reference values of foundation design. (Tales 6.1)

Useful for small structure at sites with good soils (Column Loads

< 200kN).

- Presumptive bearing pressures vary considerably, due to differing degree

of conservatism and reflecting the subsurface conditions in the area

where the code is used.

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3) Failure modes

� General shear failure →

� Local shear failure →

� Punching shear failure →

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� Failure modes →a function of relative density and relative depth(Df /B)

Bo=B for circular (diameter) or square ft .

Bo = 2BL/(B+L) for rectangular ft.

� Ultimate load occurs at 4∼10% of B for general shear failure

15∼25% of B for local-punching shear failure

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4) General Bearing Capacity Equation

qu = ultimate bearing capacity (stress)

qa = allowable bearing capacity = ../ SFqu

* Failure zones for strip footing.

Ⅰ : Active Rankine Zone

Ⅱ : Radial Zone

Ⅲ : Passive Rankine Zone

'φα = for Terzaghi

2/'45 φ+= ⇒ realistic value

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* Bearing Capacity Equation

� Presented by Terzaghi.

γγBNqNcNq qcu2

1++=

γ : unit weight of soil

c : cohesion of soil

q = γDf

Nc, Nq, Nr => Bearing capacity factors = f(φ’) � based on 'φα = (Terzaghi, Table 3.1 (p.129))

� based on 2/'45 φα += (Vesic, Table 3.4(p138))

* Assumptions

1.

2.

3.

4.

5.

6.

7.

8.

Cohesion term

Surcharge term

Friction term

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* General Bearing Capacity Equations (Bearing Capacity Factors and Other

Influential Factors)

- To consider the influences of the shape of foundations, embedded

depth, and inclined load, the following form of equation has been

suggested.

idsqiqdqsqcicdcscu FFFBNFFFqNFFFcNq γγγγγ2

1++=

i) cN , qN , rN

cN by Prandtl (1921)

qN by Reissner (1924)

rN by Caquot and Kerisel (1953) and Vesic (1973)

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ii) Shape factor (De Beer (1970))

L

BF

L

BF

N

N

L

BF

s

qs

c

q

cs

4.01

tan1

1

−=

+=

+=

γ

φ ’

where L = length of the foundation (L > B)

iii) Depth factor (Hansen(1970))

For 1/ ≤BD f , For 1/ >BD f

BDF fcd /4.01+= )/(tan4.011 BDF fcd−+=

( )B

DF

f

qd

2'sin1'tan21 φφ −+= ( )

−+= −

B

DF

f

qd12 tan'sin1'tan21 φφ

=dFγ 1 =dFγ 1

iv) Inclination Factor (Hanna and Meyerhof(1981))

2

2

)'

1(

)90

1(

φβ

β

γ −=

−==

i

qici

F

FF

where β = inclination of the load on the foundation with respect to the

vertical.

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- For inclined load, we must check sliding failure in addition to bearing

capacity.

P vP

HP

Passive

Resistance s = interface strength

(Ignored)

For clays, uss α= (generally 2/5000.1,0.1 ftlbsforbut u ≤=≤ αα )

For sands, inttan)/( φAreaPs v= )3/2( int φφ =

../)( SFAreasPH ×≤

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* Effect of water (influence on q and γγγγ of bearing capacity equation)

I) D1 < Df ( )[ ] qwsattq NDDqN γγγ −+= 21

{ } γγ γγγ BNBN wsat −=2

1

2

1

II) 0 ≤ d ≤ B qftq NDqN γ=

[ ] γγ γγγγγγ BNB

dBN wsattwsat

−−+−= )(

2

1

2

1

γt : Total unit weight of soil

γsat : Saturated unit weight of soil

γw : Unit weight of water

III) d ≥ B No effect

Note) No seepage force

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* Effect of Compressibility

For compressible soils ⇒ local shear failure

Ex) Loose sands ⇒ low relative density

- Shear (triaxial) test response and load-settlement curve for loose soil

with those for dense sand

- Terzaghi Recommendations

Use c*, φ* (reduced strength parameter) in bearing capacity

equation ;

'3

2* cc =

( )'tan3

2*tan φ=φ

εa

dense

loose

σ1-σ3

S

General failure

q (u) q(u)

P

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* Net ultimate bearing capacity

(assuming soilconc γγ = )

fu DAQq γ+= /

funet DqAQq γ−==∴ /

* Factor of Safety

i) ../ SFqq uall = , ../)()( SFqqq unetall −=

ii) sheardsheardshear FSFSccFS /,/ φφ ==⇒

idsqiqdqsqcicdcscdall FFFBNFFFqNFFFNcq γγγγγ2/1++=⇒ ,

qqq allnetall −=)(

F.S. = 3-4, 6.14.1 −=shearFS

Q

fD

qu

q = γ fDγ

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* Case history

: Ultimate Bearing Capacity in Saturated Clay

- Comparison between measured and estimated bearing capacities on 5

different sizes of the foundations.

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